RU 2 205 372 C1 describes a device for determining the fill level of a liquid in a container. This device comprises a piezoelectric transducer 1 that is acoustically coupled to the wall of the container, a first detector 2, a matching transformer 3, a first capacitor C1, an RF cable 4, a second capacitor C2, a sweep generator 5, a second detector 6, a third capacitor C3, an amplifier 7, two comparators 8 and 9, and a display unit 10. The center frequency of the sweep generator corresponds to the resonance if the wall thickness is a whole number of half-waves of the acoustic oscillations. Under this condition, the maximum amplitude of the oscillations of the circuit that consists of the secondary winding of the matching transformer and the capacitance of the piezoelectric element, is dependent on the presence or absence of liquid behind the wall. This known device is a very elaborate construction; it is inaccurate and temperature-dependent.